There has been active and extensive research into the problems of automatic or robotic assembly of parts into finished products. One of the most frequently performed part mating tasks is the insertion of pegs or pins into holes without "jamming", i.e., causing the peg or pin to become stuck in an askew position in the hole. Consequently, this task has received much scrutiny and attention. (See "Nevins, J. L., and Whitney, D. E.", "Robot Assembly Research and Its Future Applications," General Motors Research Symposium on Computer Vision and Sensor Based Robots, Warren, Mich., Sept. 25-26, 1978.)
While the problem is simply stated, i.e., how to automate the insertion of a peg into a hole without jamming; the solution is not as easily attained.
Research has shown that:
(a) for a given friction coefficient between peg and hole, assembly without jamming requires that a certain relationship between the radial-to-axial contact force ratio and the jamming moment-to-axial force ratio must be satisfied; and
(b) robotic assembly of simple rigid machine pieces, without adaptability, i.e., without some means for controlling the assembly forces and moments, has only erratic success because of forces and moments induced by unspecified compliances in the robot or the assembly fixtures.
A passive adaptive insertion device, called the Remote Center Compliance (RCC) was developed by Drake and Watson (See Drake, S. H., "The Use of Compliance in a Robot Assembly System," IFAC Symposium on Information and Control Problems in Manufacturing Technology, 1977 and Watson, P. C., "A Multidimensional System Analysis of the Assembly Process as Performed by a Manipulator", 1st North Am. Robot Conf., 1976.) This device makes possible chamfered peg-hole insertions with small (even negative) clearances, 1 mm (0.040") or larger initial position error, and a few degrees of misalignment. The RCC substantially reduces the burden on a robotic insertion system for mechanical accuracy and resolution. The proper functioning of a robot-held RCC, however, requires that the compliance of the RCC be much larger than that of the robot. This condition may be violated by large robots, or by robots with many degrees of freedom (axes).
Active adaptive insertion assembly devices have also been developed (See U.S. Pat. Nos. 3,906,325, 4,243,923 and 4,445,273) wherein a beveled peg is moved in the direction of hole insertion until contact occurs between the peg bevel and the edge of the hole. Any further motion in the insertion direction causes an interference force. The moment of the interference force is sensed by strain gages mounted on the peg or on the robot gripper. Active adaptive strategy uses the interference force as a feedback signal to a robot position control algorithm, which acts to reduce the interference force, thereby centering the pin.
Since interference force is utilized for the feedback signal, this specie of an active adaptive assembly system is referred to as a force interference sensor system. It suffers from several disadvantages. First, the strength of the feedback signal depends upon the amount of interference and the sensitivity of the force sensor, the sensitivity of which may not conveniently be increased without also increasing background signal noise caused by mechanical disturbances in the assembly environment. The force feedback signal, being a static variable, must therefore be filtered by a low-pass filter to improve the signal-to-noise ratio. The time required to filter a signal varies inversely with the filter bandwidth. This leads to a second disadvantage: the processing of the force feedback signal will be slow whenever low frequency environmental noise sources are significantly present. Finally, the force interference system is susceptible to "jamming" whenever certain relationships between the radial-to-axial contact force ratio and the jamming moment-to-axial force ratio are violated. The jamming moment results from two-point contact between the pin and the hole, that is, whenever the misalignment angle .theta.=2c/L, where c is the radial clearance and L is the length of pin penetration.
Consequently, a need exists for a system for insertion of chamfered pins into holes with improved feedback signal strength adjustability and signal processing speed and which has the potential of alleviating or reducing the tendency for misaligned pins to jam in holes.